Problems associated with rotatable tools in the form of drills for the machining of, above all, workpieces of metal, form the basis of this invention. More precisely, problems have been observed in the type of indexable-insert tools disclosed in International Patent Publication Nos. WO 03/099494 and WO 03/099495, and commercially available under the trademark CoroDrill 880®. In just this type of indexable-insert drill, the workpiece is entered initially by a radially outer part edge included in an individual main edge of a center insert, the part edge generating a surface of revolution in the form of a ring-shaped groove in the workpiece. Each infinitesimal section of this groove is passed afterwards, viz. after further rotation of half a revolution of the drill, by a peripheral cutting insert, which—in order to completely machine the entire cross-section of the hole to be made—partly overlaps the sweep area of the center insert, i.e., an inner part of the active main edge of the peripheral cutting insert extends a distance into the already generated groove.
The center insert of the known drill is formed with main edges, more precisely four main edges, each of which has a Z-like shape by including two mutually displaced part edges, the radially outermost one of which is located a distance in front of the part edge situated innermost as viewed in the axial feeding direction of the tool. The individual peripheral cutting insert, which generally enters and machines the workpiece at a stage after the outer part edge of the center insert, but before the inner part edge thereof, is however, in practice, formed with straight main edges, the edge lines of which extend linearly between surface-wiping secondary edges adjacent to corners of the cutting insert. The cutting inserts have generally a positive cutting geometry so far that the chip surface positioned inside the edge line, as well as a possibly occurring chamfer surface (reinforcement bevel), forms an acute angle with the clearance surface of the cutting insert.
The aforementioned problems manifest themselves in that chippings and cracks arise in the peripheral cutting insert, i.e., the cutting insert that afterwards intersects the groove-shaped surface of revolution that already has been generated by the outer part edge of the center insert. Such crack formation may at times develop into drill breakdowns.
Problems with chipping and crack formation may also be found in other types of cutting tools, in particular rotatable cutting tools, of the type that includes two or more cutting inserts operating in sequence one after the other, the sweep areas of which overlap each other, more precisely in the subsequent cutting insert that, with a certain time delay, intersects a surface already generated by a preceding cutting insert. As non-limiting examples of such tools, milling cutters in the form of cylindric cutters and boring tools, respectively, are included.